1. Field of the Invention
The invention relates generally to side chain liquid crystalline polymers exhibiting nonlinear optical responses. More particularly, the side chain polymer is made up of pendant groups attached via flexible spacers to a polymer backbone. Chiral structures in the polymer backbone or in other pendant groups give the polymer non-centrosymmetry while the nonlinear optical and liquid crystalline properties come primarily from the pendant groups. The organic, polymeric, nonlinear optical, and liquid crystalline properties may all exist simultaneously within a single physically homogeneous medium.
2. Description of the Related Art
Nonlinear optics has become significant in recent years due to the telecommunications industry's search for materials capable of high-speed optical signal processing, the development of increasingly sophisticated laser technology, and the possibilities of using fiber optic technology in computers. Nonlinear optics deals with the interactions of incident electromagnetic fields with various substances, such as thin films, crystals, or fiber strands. The resultant optical fields can be altered from the incident fields in phase, frequency, amplitude, and other propagation characteristics.
It is known in the art that certain organic and polymeric materials exhibit greater nonlinear optical (nlo) response than inorganic crystals. As organic materials, they are resistant to damage by powerful lasers and have large dipole moment changes upon pi-electronic excitation. They also offer a great variety of molecular structures to optimize nlo response. Further, as polymers, they have advantages such as improved mechanical strength, processability, and the capacity to form thin films of high optical quality. Inexpensive lenses, prisms, anti-reflection coatings, and fiber optics are but a few of the plastic components that can be easily produced through the use of polymers.
It has been demonstrated that large delocalized pi-electron systems are largely responsible for the nlo response in organic and polymeric materials. The presence of pi-donor and pi-acceptor groups conjugated through an extended molecular framework is conducive to very rapid response times upon pi-electronic excitation. Aromatic nitro compounds in which a pi-electron donor such as oxygen or nitrogen is conjugated with the pi-electron acceptor nitro is one species having such large delocalized pi-electron systems. Polymeric compounds, such as polyacetylenes, also possess conjugated pi-electron systems, but do not necessarily have pi-donor and pi-acceptor groups.
Organic-based nlo materials hold great promise for use in various optical communications technologies such as high-speed optical switches and optical fibers. Other applications include optical wave guides for high resolution lithography, frequency doublers for semiconductor lasers, and optical signal processing. The molecular engineering and desired geometric and electronic features of new nlo materials have a complementary aspect in that many of the ultrastructural considerations are required for, or conducive to, the design of organic ferroelectric and piezoelectric materials.
One disadvantage of existing organic and polymeric materials is that multicomponent systems, such as "guest-host" systems, must be used in order to achieve both nonlinear optical and liquid crystalline properties in a single material. It is common to use a physical mixture of components, e.g., a polymer and a chromophore. A chromophore (dye) monomer "guest" must be added to an ordinary liquid crystalline polymer "host" to produce a multicomponent and possibly multiphase system.
Another disadvantage of existing organic and polymer nlo materials is that electric field poling is necessary to align the molecules and dipoles.
As discussed previously, organic and polymeric molecules derive their nonlinear optical properties from pi-electronic interactions within the molecule. Pi-electronic interactions vary with the molecular structure. This nonlinear optical behavior can be expressed in terms of the dipolar approximation with respect to the polarization induced in an atom or molecule by an external field. Twieg and Jain, using the fundamental equation (1), explain that in the dipolar approximation the change in dipole moment, .DELTA.,.mu., of an individual molecule upon interaction with the electric component of electromagnetic radiation is described as a power series of the electric field strength E as given by equation (1) where .mu..sub.e and .mu..sub.g are the molecular excited state and ground state dipole moments, respectively.
The coefficient .alpha. is the linear polarizability; .beta., the quadratic hyperpolarizability; and .gamma., the cubic hyperpolarizability, etc. EQU .DELTA..mu.=.mu..sub.e -.mu..sub.g =.alpha.E+.beta.EE+.gamma.EEE+(1) EQU P=P.sub.o +.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+(2)
Upon scaling from a single molecule to an array of molecules, i.e., a crystal, the appropriate expression becomes that in (2) where P is the macroscopic polarization and the meaning of the coefficients .chi..sup.(1), .chi..sup.(2) and .chi..sup.(3) is similar to their counterparts .alpha., .beta. and .gamma. in the microscopic description. Equation (2) is identical with (1) except that it describes a macroscopic polarization, such as that arising from an array of molecules in a crystal. The odd order coefficients are not symmetry dependent and are always non-vanishing. Even order coefficients, however, are dependent on symmetry and go to zero for centrosymmetric materials. The quadratic hyper-polarizability .chi..sup.(2) governs second harmonic generation (SHG). Thus, a material must be non-centrosymmetric to provide SHG. The odd order coefficient .chi..sup.(3) is responsible for third harmonic generation (THG).
Coherent light waves passing through an array of molecules can interact with them to produce new waves; this interaction may be interpreted as resulting from a modulation in refractive index or alternatively as a nonlinearity of the polarization. Such interaction occurs most efficiently when certain phase matching conditions are met, requiring identical propagation speeds of the fundamental wave and the harmonic wave. Birefringent crystals often possess propagation directions in which the refractive index for the fundamental .omega. and the second harmonic 2 .omega. are identical so that dispersion may be overcome.
Referring generally to liquid crystalline technology, it is known that thermotropic liquid crystals are prepared by heating. When the solid is heated, it transforms into a turbid system that is fluid and birefringent. Upon cooling, the material converts back from isotropic liquid to liquid crystal to solid. Side chain liquid crystalline polymeric substances are able to align with the major axes of pendant groups statistically parallel over a significant distance. While in a liquid crystalline phase the molecules can be easily aligned uniformly over a still greater distance, that is, macroscopically aligned with an external electromagnetic field. The molecules may then be frozen into a glassy phase upon cooling.
This liquid crystalline (mesomorphic) state of matter has the ability to combine long range as well as short range characteristics. These liquid crystals exist in two major structural arrangements or phases. The two phases, nematic and smectic, are each characterized by parallelism of the major molecular axes. The nematic phase allows for translational mobility of constituent molecules, and when heated, generally transforms into the isotropic phase. The smectic phase is composed of molecular layers in which translational mobility is minimal.
There are a variety of smectic phases differing in the ordering of molecules within the same layer, the tilt of the "average" molecular axis with respect to the layer plane, and the positional correlation of molecules in different layers. A common example is the smectic type A. Recently, the chiral smectic C phase has been shown to be ferroelectric. Another development is a working electro-optical device based on coupling the spontaneous polarization to an applied electric field. Also, there is a particular liquid crystal known as the smectic D, which not only shows a three-dimensional ordering but is also optically isotropic. A further structural arrangement, the cholesteric phase, is locally similar to the nematic phase, but is composed of chiral molecules, i.e., a chiral nematic.